Is it possible to convert image texture's black color to transparent?
Yes.

First Cycles  Texture Non Color then Mix Transparency and Emission.



Nodes



You might consider processing your images once as format .png for you later convenience and node convenience.

Second a sample from Blender Render. UV Mapped Image. Texture 01 Image .. calculate transparency.  Threshold with color ramp gray scale . Affect color and transparency. Texture 02 color only multiply. You may want to threshold because the image may not be black exactly where you might first think.  There are many was to do this.  Torus Background and Text to show transparency.



2 Textures. Note the color ramp.